Injection molding process
- Nov 24, 2018 -

The process flow


These 6 stages directly determine the forming quality of products, and these 6 stages are a complete continuous process.



Folding and filling stage


Filling is the first step in the whole process of injection molding. The time from the closing of the mold to the filling of the mold cavity is about 95%. Theoretically, the shorter the filling time, the higher the forming efficiency; But in actual production, molding time (or injection speed) is subject to many conditions.



High speed filling. When filling at high speed, the shear rate is high, and the viscosity of the plastic decreases due to the thinning of shear, which reduces the overall flow resistance. The local viscous heating effect will also thin the curing layer thickness. Therefore, in the flow control stage, the filling behavior often depends on the volume to be filled. That is to say, in the flow control stage, due to high-speed filling, the shear thinning effect of melt is often very large, while the cooling effect of thin wall is not obvious, so the effect of speed prevails.



Low speed filling. When heat conduction controls low filling speed, the shear rate is low, the local viscosity is high, and the flow resistance is large. Due to the slow rate of hot plastic supplement and slow flow, the heat conduction effect is obvious, and the heat is quickly taken away for the cold mold wall. With a small amount of viscous heating, the thickness of the curing layer is thicker, which further increases the flow resistance at the thinner part of the wall.



Due to the fountain flow, the plastic polymer chain in front of the flow wave rows towards almost parallel flow wavefront. Therefore, when the two strands of plastic melt glue meet, the polymer chains on the contact surface are parallel to each other. In addition, the properties of the two strands of melt adhesive are different (the retention time in the mold cavity is different, and the temperature and pressure are also different), resulting in poor structural strength of the melt adhesive intersection region on the micro level. In the light of the parts placed in the right Angle with the naked eye observation, you can find that there are obvious joint line, this is the formation mechanism of weld marks. Weld marks not only affect the appearance of plastic parts, but also their microstructure is loose, easy to cause stress concentration, so that the strength of this part decreases and fracture occurs.



Generally speaking, the weld mark strength is better in high temperature area. In addition, the temperatures of the two melt strands in the high-temperature zone are close to each other. The melt's thermal properties are almost the same, which increases the strength of the fusion zone. Conversely, in the low temperature area, the welding strength is poor.

The holding period


The role of the holding stage is to continuously apply pressure, compaction melt, increase the density of plastic (densification), to compensate for plastic shrinkage behavior. In the process of holding pressure, the back pressure is high because the mold cavity has been filled with plastic. In the process of compaction, injection molding machine screw can only slowly forward for small movement, plastic flow speed is also relatively slow, at this time the flow is called pressure flow. Due to the pressure stage, the plastic by the mold wall cooling curing speed, melt viscosity increases quickly, so the resistance in the mold cavity is very large. In the later stage of pressure retaining, the material density continues to increase and the plastic parts gradually take shape. The pressure retaining stage should continue until the gate solidifies and seals. At this time, the cavity pressure in the pressure retaining stage reaches the maximum.



In the stage of holding pressure, due to the high pressure, plastic presents a part of compressible characteristics. In the high pressure area, the plastic is relatively dense and dense. In the area with low pressure, the plastic is loose and has low density, so the density distribution changes with location and time. In the process of pressure retaining, the plastic flow rate is very low, and the flow no longer plays a leading role; Pressure is the main factor that affects the process of holding pressure. During the process of holding pressure, the plastic has been filled with mold cavity. At this time, the gradually solidified melt ACTS as a medium to transfer pressure. The pressure in the mold cavity is transferred to the surface of the mold wall by means of plastic, which tends to open the mold. Therefore, appropriate clamping force is required for the clamping. Under normal circumstances, the rising die force will slightly push the mold open, which is helpful for the exhaust of the mold. But if the mold force is too large, it is easy to form the rough edge, overflow, or even open the mold. Therefore, in the choice of injection molding machine, should choose a large enough clamping force injection molding machine, in order to prevent the phenomenon of swelling and effectively pressure.



In the new injection environment, we need to consider some new injection molding technology, such as gas-assisted molding, water-assisted molding, foaming injection molding and so on



Folding cooling stage


Cooling system design is very important in injection mold. This is because molding plastic products only cooling solidification to a certain rigidity, after demolding to avoid plastic products due to external forces and deformation. Since the cooling time accounts for about 70%~80% of the whole molding cycle, a well-designed cooling system can significantly shorten the molding time, improve injection molding productivity and reduce costs. Improper cooling system will prolong the molding time and increase the cost. Uneven cooling will further cause plastic warping deformation.



According to the experiment, the heat from the melt into the mold is roughly divided into two parts, one part of which 5% is transferred to the atmosphere by radiation and convection, and the other 95% is transferred from the melt to the mold. Plastic products in the mold due to the role of the cooling water pipe, the heat from the plastic mold cavity through heat conduction through the mold frame to the cooling water pipe, and then by thermal convection is carried away by the coolant. A small amount of heat that is not carried away by the cooling water continues to be transferred in the mold and is dispersed in the air after contact with the outside world.



The molding cycle of injection molding consists of closing time, filling time, holding time, cooling time and demoulding time. The cooling time accounted for the largest proportion, about 70%~80%. Therefore, the cooling time will directly affect the length of plastic molding cycle and output. The temperature of plastic products in the demolding stage should be cooled to lower than the thermal deformation temperature of plastic products to prevent the relaxation of plastic products caused by residual stress or warping and deformation caused by external demolding force.



The factors influencing the cooling rate of products are as follows:



Plastic product design. Mainly is the plastic product wall thickness. The thicker the product, the longer the cooling time. Generally speaking, the cooling time is in direct proportion to the square of the thickness of plastic products or the 1.6 square of the maximum diameter of the flow channel. Namely plastic product thickness doubles, cooling time increases 4 times.



Mould material and its cooling method. Mold material, including mold core, mold cavity material and mold frame material has a great impact on the cooling speed. The higher the thermal conductivity of the mold material, the better the effect of heat transfer from the plastic per unit time, and the shorter the cooling time.



Cooling water pipe configuration. The closer the cooling water pipe is to the mold cavity, the larger the diameter and number of pipes are, the better the cooling effect is and the shorter the cooling time is.



Coolant flow. The larger the flow rate of cooling water (turbulence is generally preferred), the better the effect of cooling water on heat removal by thermal convection.



Properties of coolant. The viscosity and heat conduction coefficient of the coolant also affect the heat conduction effect of the mold. The lower the viscosity of coolant, the higher the thermal conductivity, the lower the temperature, the better the cooling effect.



Plastic selection. Plastic is a measure of the speed at which plastic transfers heat from a hot place to a cold place. The higher the heat conduction coefficient of plastic, the better the heat conduction effect, or the lower the specific heat of plastic, the temperature is easy to change, so the heat is easy to dissipate, the better the heat conduction effect, the shorter the cooling time.



Processing parameter setting. The higher the material temperature, the higher the mold temperature, the lower the ejection temperature, the longer the cooling time required.



Design rules for cooling system:



The designed cooling channel should ensure that the cooling effect is uniform and rapid.



The cooling system is designed to maintain proper and efficient cooling of the mold. Cooling holes shall be of standard size for easy machining and assembly.



When designing the cooling system, the mold designer must determine the following design parameters according to the wall thickness and volume of the plastic parts -- the position and size of the cooling hole, the length of the hole, the type of hole, the configuration and connection of the hole, and the flow rate and heat transfer property of the cooling fluid.



Folding and demoulding stage


Demoulding is the last step in an injection molding cycle. Although the products have been cold solid forming, but demolding or on the quality of products has a very important impact, demolding mode is not appropriate, may lead to products in demolding force uneven, jacking out of the product deformation and other defects. There are two main ways of demoulding: ejector rod demoulding and stripper plate demoulding. In order to ensure the quality of the products, we should choose the appropriate demoulding method according to the structural characteristics of the products.



For the selection of ejector rod demoulding mold, ejector rod setting should be as uniform as possible, and the location should be selected in the ejector rod resistance and the maximum strength and stiffness of the plastic parts, so as to avoid plastic parts deformation damage.



The stripper plate is generally used for deep cavity thin-wall containers and transparent products that do not allow push rod marks. The mechanism is characterized by large and uniform stripper force, smooth movement, and no obvious traces left behind.



Fold and edit the process parameters of this section


Folding injection pressure


The injection pressure is provided by the hydraulic system of the injection system. The pressure of the hydraulic cylinder by injection molding machine screw is passed on to the plastic melt, under the impetus of the plastic melt in the pressure, the injection molding machine nozzle into the mold of the vertical flow channel (for part of the mould is also the mainstream way), mainstream channel, distributary channel, and the gate into the mold cavity, the process is the injection molding process, or filling process. Pressure exists to overcome resistance in melt flow process, or conversely, the resistance in flow process needs to be offset by pressure of injection molding machine to ensure smooth filling process.



During the injection molding process, the pressure at the nozzle of the injection molding machine is the highest to overcome the flow resistance in the whole melt process. After that, the pressure decreases gradually along the flow length to the front wave front of the most front-end of the melt. If the exhaust inside the mold cavity is good, the final pressure at the front end of the melt is atmospheric pressure.



There are many factors influencing melt filling pressure, which can be summarized into three categories :(1) material factors, such as the type of plastic, viscosity, etc. (2) structural factors, such as the type, number and location of the casting system, mold cavity shape and product thickness; (3) forming process elements.



Folding injection time


The injection molding time mentioned here refers to the time required for the plastic melt to fill the mold cavity, excluding the auxiliary time for mold opening and closing. Although the injection time is very short and has little impact on the molding cycle, the adjustment of the injection time has a great effect on the pressure control of the gate, runner and cavity. Reasonable injection time is helpful for the ideal filling of melt and is of great significance for improving the surface quality of products and reducing the dimensional tolerance.



The injection molding time is much lower than the cooling time, which is about 1/10~1/15 of the cooling time. This rule can be used as the basis for predicting the whole molding time of the plastic parts. In the case of mold flow analysis, the injection time in the analysis results is equal to the injection time set in the process conditions only when the melt is completely driven by screw rotation to fill the cavity. If the pressure retaining switch of the screw occurs before the cavity is filled, the analysis result will be greater than the set process conditions.



Folding injection temperature


Injection temperature is an important factor affecting injection pressure. Injection molding machine cylinder has 5~6 heating sections, each raw material has its appropriate processing temperature (detailed processing temperature can refer to the data provided by the material supplier). Injection molding temperature must be controlled within a certain range. The temperature is too low, the melt plasticization is bad, affects the quality of the molding parts, increase the difficulty of the process; The temperature is too high for the raw material to decompose easily. In the process of practical injection molding, injection molding temperature are often higher than the cylinder temperature, higher values associated with injection rate and material performance, up to 30 ℃. This is because the melt through the injection port by shear and produce high heat. There are two ways to compensate for this difference in mold flow analysis: one is to try to measure the temperature at which the molten material is injected into the air; the other is to include the nozzle in the modeling.



Folding pressure holding pressure and time


At the end of the injection molding process, the screw stops rotating and just pushes forward. At this point, the injection molding enters the pressure maintaining stage. In the process of pressure maintaining, the nozzle of the injection molding machine continuously fills the material into the cavity to fill the volume vacated due to the shrinkage of the parts. If the cavity is full of pressure, the parts will shrink about 25%, especially because the tendon shrinkage is too large and the formation of contraction marks. Holding pressure is generally about 85% of the maximum pressure of filling, of course, according to the actual situation to determine.



Fold back pressure


Back pressure is the pressure to be overcome when the screw reverses and retreats. The use of high back pressure is conducive to the dispersion of color and plastic melting, but at the same time to extend the screw shrinkage time, reduce the length of plastic fiber, increase the pressure of the injection molding machine, so the back pressure should be lower, generally not more than 20% of the injection pressure. When injecting foam, the back pressure should be higher than the pressure formed by gas, otherwise the screw will be pushed out of the barrel. Some injection molding machines can program back pressure to compensate for reduced screw length during melting, which reduces heat input and lowers temperature. However, because the results of such changes are difficult to estimate, it is not easy to adjust the machine accordingly.